Optical coupler

ABSTRACT

A reflective star coupler for a distributed switch optical network for narrow band and broad band broadcast communication incudes on port terminated in a reflective surface or device, a signal amplifier and/or filter is disposed in the path between the port and the reflective device to reintroduce the light from that port into the coupler in an amplifier state or selectively according to wavelength. The amplifying device may be a semiconductor laser having a reflective coating on one face thereof.

This invention relates to an optical coupler for a communicationnetwork.

A multipoint communication network, such as a local exchange fortelephony, has conventionally used a copper wire central exchange-basedswitching network and electronic or mechanical switching. Following thedevelopment of optical fibres, optical networks have been developed. Theadvantage of an optical fibre-based network is that it can supportconsiderably more simultaneous communications and/or communications of aconsiderably greater bandwidth over the same optical fibre.

Current passive optical networks (PONs) are based on conventionalcentralised switching techniques, using electronic switching devices.Their reliance on electronic switching in an otherwise opticalfibre-based network band limits the paths in the network to the capacityof the switches, creating an `electronic bottleneck`. Typically, currentgenerations of electronic switches can support 64 kbit/s throughput.This is sufficient for a small number of simultaneous voice channels butit is inadequate for two-way switching of video as used, for example, invideo conferencing. It is also inadequate for supporting broad bandbroadcast communications, such as high definition television.

Distributed switching is known in which the paths in a network areconnected to nodal connectors. The network is broadcast based, eachcustomer being able to receive the output of the other customerterminals.

However, these are currently limited to electronic networks because ofthe potential losses associated with the optical connectors which wouldbe required at the network nodes.

U.S. Pat. No. 4,787,693 discloses a passive star optical coupler for abroadcast type local area network (LAN) in which a message transmittedby one user can be received by all others. The basic coupler is areflective star arrangement in which one port of a transmissive star isterminated in a reflective surface in order to produce a coupler whichis transparent to all terminals connected to the remaining ports.

There are, of course, various ways in which a reflective star couplercan be constructed which will be apparent to the skilled person. Anothersimple reflective coupler arrangement for a serial data bus is disclosedin U.S. Pat. No. 4,457,581.

However, as mentioned above, the main problem to be overcome is that ofthe losses in the coupler which can lead to a degradation in the signalpassing through it. It is this which limits the applicability of opticalfibre couplers to network applications in which a considerable number ofcouplers are required.

According to the present invention there is provided an optical couplercomprising a plurality of light transmissive elements communicativelycoupled at a coupling point whereby in use light transmitted in each ofthe elements is coupled into each other of the elements, one of theelements being provided with light retransmissive means for reapplyinglight leaving the coupler in that element back to the coupling point,characterised in that the said one element is also provided with signalconditioning means in the path of the light transmitted along the oneelement.

Thus, the coupler is transparent to signals in the various elements, butthe losses associated with the coupler can now be counteracted by thesignal conditioning element. Alternatively, the signal conditioningelement may be used to render the coupler transparent only atpredetermined wavelengths or ranges of wavelengths.

In one form, the signal conditioning means are an amplifier. The signalconditioning means may also be a filter or a filter/amplifiercombination.

When a filter is used it may be frequency domain or time domainselective.

The invention also extends to an optical switching network, for example,a passive optical network incorporating couplers according to theinvention. The network may be a distributed switching network in whichat least some of the nodes are constituted by the couplers.

According to another aspect of the invention there is provided anoptical signal conditioning device comprising a laser having at leasttwo light transmitting ports and characterised by reflecting meansarranged to reflect the output of one port back into the laser to beemitted from the other port.

Reference is made to optical fibres and it will be understood that thepresent invention is equally applicable to electromagnetic radiationhaving wavelengths outside the visible light band, for example infra-redand ultra-violet light.

The present invention can be put into practice in various ways some ofwhich will now be described by way of example with reference to theaccompanying drawings, in which:

FIG. 1 is a block diagram of a passive optical network;

FIG. 2 is a schematic diagram of a 2×2 star coupler according to theinvention;

FIG. 3 is a block diagram of a reflective star distributed switchnetwork incorporating an alternative form of star coupler also accordingto the invention;

FIG. 4 is a schematic diagram of a modified semiconductor laser chipaccording to the invention;

FIG. 5 is a further form of reflective coupler according to theinvention;

FIG. 6 is a block diagram of a 4×4 star coupler according to theinvention; and

FIG. 7 is a heterodyne optical tuner incorporating the invention.

Referring to FIG. 1, a conventional passive optical network localexchange comprises a central exchange trunk switch 10 which is connectedto a first local distributor 12. This, in turn, is connected to each ofa plurality of customers 14 through a set of power dividers or splitters16. The network constitutes a `tree` configuration in which the centralswitch 10 routes calls to the various customers.

In order to compete with copper wire networks, it is necessary for anequivalent optical system to have equally low, or lower, installationand maintenance costs. Thus, on a basic level it is desirable to be ableto implement an optical system to provide narrow band services, such asvoice and low speed data telephony, and yet also provide the potentialfor evolution to broad band two-way switched services within theexisting network on a higher level, since it has the inherent capacityand only the terminal equipment would need upgrading.

Referring to FIG. 2, a 2×2 optical coupler is shown in which a firstsingle mode fibre 18 is coupled, over a suitable coupling distance L, toa single mode fibre 20. The left-hand end 18a of the fibre 18 isconnected to the trunk exchange switch 10. The right-hand ends 18b and20b of both fibres 18 and 20 are connected to the power dividers 16 andthence to the customer terminals 14.

The left-hand end 20a of the fibre 20 is connected to a terminatingreflective surface 22 through an amplifier 24. The coupler is used toreplace the distributor 12 in FIG. 1. The coupler allowsinter-communication of signals on either fibre, from either end to betransmitted on the other fibre. Thus, the network functions as adistributed switch for local traffic as the signals on one fibre aretransmitted through the coupler to the other fibres in the network. Inaddition, the network still works as a tree structure to signalstransmitted from the trunk exchange switch 10 to the customers 14. As aresult, the network becomes a reflective coupler (as seen by eachcustomer) in a distributed switching network while maintaining the treestructure required by the trunk exchange switch 10. Customer access tothe network is via a single mode opto-electronic transceiver with highpeak transmission power and high sensitivity.

There are other forms of star topology, but the reflective star topologyhas the advantage of only a single optical path to each customer andbetween any pair of customers.

While the reflective star coupler could be used without the amplifier,the losses resulting from reflection and the `round trip` path affordedby the coupler are not insignificant. This problem is magnified whenconsiderable numbers of such couplers are required in a single network.The level of the losses in the network rise with the splitting ratio ofthe signal at each coupler. Thus the amplifier is provided in order toenhance the signal level and compensate for the losses.

A particular advantage of locating the amplifier at the reflection isthat signals make a double pass through the amplifying device and adouble amplification is obtained.

One particularly suitable amplifier is a semiconductor laser device. Inthis case, the reflecting medium can take the form of a coating on thedevice and only a single fibre connection is required. Thus, aconventional single fibre device can be used. A suitable semiconductorlaser device is discussed in more detail below.

FIG. 3 illustrates a modified network in which the reflection is madefrequency selective by the use of a filter 26. Of course, the sameeffect would be achieved by using a wavelength selective amplifier (i.e.an active filter) as opposed to the separate amplifier and filter unitillustrated. In either case the customer `sees` the reflective networkacross a selected waveband but not at other wavelengths. In all otherrespects, the network of FIG. 3 is similar to that in FIG. 1, adapted inaccordance with FIG. 2, to constitute a distributed switch network, andlike reference numerals have been used to denote the same elements.

As an alternative to wavelength filtering it would also be possible toset up a selectively reflecting arrangement based on a synchronised timedivision multiple access principle. This would require time slotallocation to customers, or groups of customers, eligible to accesscertain signals in selected time slots.

Both semiconductor lasers and fibre lasers are considered suitable twoport light amplifying devices in order to amplify low level signals inthe reflective arm of the coupler. These devices are normallybi-directional and can, therefore, be used between two terminals toamplify signals in both directions.

A reflective laser amplifier reflector can be made for use in thepresent invention by adding a reflective mirror or coating to one portof a semiconductor laser device, as shown in FIG. 4.

The device comprises a semiconductor laser chip 27 conventionallypowered from an electrical input 28 on one terminal and having a groundconnection 30 to the substrate. A high reflectivity coating 32 isapplied to one end of the chip in order to produce substantially totalreflection at that end. The other end of the chip is aligned with atapered lens ended optical fibre which constitutes the fibre 20 in FIG.2. An anti-reflective coating 33 is preferably applied to the emittingend of the laser chip 27.

This effectively one port optical device can, therefore, be used as theamplifying reflective node of a reflective star coupler in a distributedswitch network. The device can also be arranged as a filter havingwavelength selection by applying a wavelength dependent multi-layerreflective coating, responsive to predetermined ranges of wavelengths oflight to either of the faces of the device to admit or reflect light,respectively, according to the selected wavelengths. Alternatively, thelaser chip itself can be arranged to be responsive to only apredetermined range of wavelengths.

When used as a reflective node, with gain, a semiconductor laseramplifier such as this would normally be biased below its emissionthreshold. It could also be used to transmit signals into the network bybiasing above the threshold.

In time division multiplex (TDM) systems, this added signal facilitycould be used for network synchronisation and/or control and wouldnormally have a low duty cycle.

The gain of the amplifier could also be varied by varying the biascurrent. This facility may be particularly useful in the event offailure of other methods of signal modulation and/or control or to adddownstream signals in a customer terminal's time slot.

The laser chip could be a broad band device structure, such as a buriedheterostructure, or a narrow band device, such as a distributed feedbacklaser.

The device structure and packaging is very similar to that used forconventional, unmodified semiconductor lasers. Consequently, single portamplifiers of this type can be packaged using conventional productionmethods which could means they are as cheap to produce as a conventionalsemiconductor laser source.

A fibre laser amplifier-based coupler reflector is illustrated in FIG.5. Signals from the network pass through a filter 34 and a 50% coupler35, in both directions, by means of a doped fibre loop 36.

Power is provided by an energy pump 38 via a filter 40 to the coupler.The pump energy enters the doped fibre loop 36 at a level which permitssignal gain to occur through photon interaction in the doped region. Theamplified signal then passes out of the loop through the coupler and thefilter 34 back into the network. The coupler 35 and doped loop 36constitute an amplifier/reflector in which the light in the limb isreadmitted via the loop and coupler (which couples together the ends ofthe loop) to the limb 20a.

The filter 34 is required to prevent pump energy from entering thenetwork and/or to select wavelengths for amplified retransmission. Thus,in part, it serves the same function as the filter 26 in FIG. 3.

The filter 40 is required to prevent reflection of the signal at the endof the fibre connected to the pump 38. Ideally, the filters 34 and 40would not be required in a situation in which no wavelength filteringwas used as the coupler would split power evenly at both the pump andsignal wavelengths. Thus, the arrangement would be sufficiently wellbalanced to prevent signal energy arriving at the pump or pump energybeing delivered to the network fibre.

Another form of reflecting termination for the fibre 20a is a singlemode fibre loop connected to a coupler as is shown in FIG. 5. However,in this case, the filter 40 and pump 38 are dispensed with and the fibreto the right of the coupler can be left unterminated, or properlyterminated if the coupler is unbalanced, as necessary. Again, the filter34 is optional and can be included if the reflective coupler is to bewavelength selective. In this reflective coupler a bi-directionalsemiconductor laser chip is inserted in the loop to amplify the lightpassing into the loop and out via the coupler.

The manner in which the network is accessed by customers depends onwhether the network is to be operated in the narrow band or broad bandmode.

For narrow band services a network protocol could be designed to suit 64kbit/s telephony and would be TDMA based. Broad band access could beeither TDMA or wavelength division multiple access (WDMA). For narrowband services a synchronous rather than asynchronous TDMA system isbandwidth efficient and would result in relatively cheap electronics inthe customer terminals. The cost of the system per customer would beroughly evenly split between the cost of the fibre, opto-electronictransceiver and customer access electronics. Installation costs could beminimised using blown fibre techniques and low cost polythene ducts.

A synchronous TDMA system depends upon obtaining an accurate clock towhich all other clocks may be locked. This clock would ideally be amultiple of 8 kHz, which is the sampling rate for conventional telephonybandwidth signals. A master clock source would normally be found in thetrunk exchange. This clock signal could be broadcast to all customers asa sequence of narrow, i.e. low duty cycle, pulses. The chosen sequencecould be pseudo-random binary sequence or a Barker sequence which has alow probability of being mimicked by traffic on the network.

The clock in the customer's equipment can be delay-locked to theincoming sequence. Each customer is allocated a unique periodic timeslot following the synchronisation pulse, which serves as a destinationaddress. The customer's equipment contains a variable delay line whichallows the customer to transmit pulses in any predetermined vacant timeslot and communicate with other terminals via the reflective node. Eachcustomer's terminal receives the sync. pulses at different timesdepending upon the optical path length from the reflective node. Theround trip propagation delay must be determined and subtracted from thevalue stored in the variable delay line memory so that the correctdestination time slots can be accessed. In a transparent network, suchas a distributed switch network, the customer terminals will also bedifferent distances from the low duty cycle reference pulses at the nodesent out to lock all customers to that reference from the trunkexchange. It is therefore necessary to account for the delay in atransmitted message for another customer in reaching the distributivenode to coincide with the time slot specific to the customer addressed.

Thus, the time at which one customer transmits must take account of theround trip to the customer addresses.

To overcome this each terminal is provided with a delay time memoryspecific to its distance from the reflective node. This is accessed fora particular customer and the appropriate adjustment made for thetransmitted message to meet the time slot at the reflective node.

The round trip delay could be determined when the customer's signal isconnected to the network. To do this, an upstream signal would betransmitted in a way which minimises interference with other users. Forexample, a pseudo-random binary sequence is transmitted at a lowamplitude so that it is below the noise threshold of other receivers onthe network. A correlation detector would then be used to determine theround trip delay. The main complication with this technique is that thecustomer transceiver would be required to switch from transmit toreceive rapidly during ranging to recover enough of the sequence to makean accurate correlation.

An alternative method is to operate the system clock rate at a slightlyhigher rate than is strictly necessary, such as 8.08 kHz. Every 100thsync. frame of 124 micro seconds is designated as redundant and leftempty for ranging. The added requirement of this approach is thatadditional timing and memory circuitry is required in each terminal torestore the 8 kHz reference data rate.

A further alternative method would be to send ranging pulses at a timewhen the traffic on the network is low so that the number of error bitsdetected by the customers' receivers is within set limits, for example 1bit in 100,000.

A detailed call handling procedure is required to ensure calls arecorrectly received and transmitted. The essential features are asfollows:

Incoming trunk calls are switched into the time slot of the destinationterminal by the local exchange only if it is found to be empty for apredetermined time, e.g. 250 microseconds;

Some time slots are allocated to trunk call servers in the localexchange. Outside calls are made by accessing an empty trunk time slot;

Internal calls are made by accessing the time slot corresponding withthe address of the terminal being called;

Common channel signalling procedures are implemented by designing thenetwork to have an additional 8 kbit/s data channel with a 64 kbit/schannel. Most of the special features required in a private branchexchange (PBX) could be implemented using software resident in eachterminal via this additional communication channel; and

An address memory is required in each terminal which would be updated aseach terminal is added.

The network size is limited by the type of reflective node(s) used andthe optical power budget of the customer transceivers. The transmittersoperate with a duty-cycle approximately equal to the split ratio andcould, therefore, operate with a higher peak power than their normalcontinuous rating. This compensates for the loss of up to one passthrough the network and ensures that the received power levels from thetrunk exchange and customers are approximately equal. Initially, thenetwork need not be fully utilised but sufficient power dividers wouldbe needed to allow for growth.

The receiver sensitivity is inversely proportional to the bit rate andcorresponding split ratio. If the split ratio is doubled, the round triploss is quadrupled and the receiver sensitivity is halved. This poses asevere constraint on network expansion beyond a certain split ratiounless special measures are taken such as an increase in the gain of theoptical amplifier.

Other methods of increasing the split ratio include an alternative startopology and the use of call concentration.

Referring to FIG. 6, the number of reflective nodes can be increased ifmore 2×2 couplers are added to form a more complex matrix. Thisincreases the reflected power by the number of nodes added. However, themethod introduces multiple paths which would become apparent, causingcorrupted signals, at very high bit rates and with coherent systems. Thematrix in FIG. 6 is a 4×4 reflective coupler comprising fourcross-connected 2×2 couplers 43 and similar to that in FIG. 2. Acombined amplifier/filter 42 is connected to each reflective leg 44.

The use of trunking on the network itself allows call concentration.Time slots are then allocated to customers on demand. A separatesignalling time slot would be required which would be common to allusers. This technique adds complexity but allows better utilisation ofthe available bandwidth. The amount of concentration would depend uponthe ratio of external to internal traffic and the grade of serviceallowed.

The nominal bit rate available to any customer is determined by therepetition rate of his time slot. However, the local network can supportmore information than a single bit in each time slot. Provided that aninternal call is made, additional bits or a pulse amplitude modulated(PAM) signal could be added in the period of the time slot, providedthat the destination terminal is able to receive the broader bandwidthsignal.

The reflective star network could also be upgraded using coherenttechnology to provide interactive broad band services in the local area.A frequency division multiplex (FDM) access protocol is establishedwhich has similar features to that for TDM. Frequency bands areallocated to receive terminals in a similar way to time slots. Thenetwork no longer requires synchronisation, but would require an opticalreference wavelength. Since it would pass only once through the networkit could be used as a local oscillator for heterodyne systems. Indeed,the optical distributed switch system is particularly applicable toheterodyne terminals as the network can make do with only one localoscillator (LO) source for all terminals. This LO is advantageouslylocated at the trunk exchange switch as a coherent source. Customerterminals would then access channels via the intermediate frequency bandof the heterodyne system terminal. In order for a transmitting customerterminal A to access a customer terminal B the terminal A must transmitat a selected optical frequency which, when mixed with the LO frequencyin the terminal B, provides the terminal B access intermediate frequencyas the difference between the two. As a practical matter theintermediate frequency is desirably significantly lower, i.e. a radiofrequency or lower, than the optical frequencies on the network in orderto lessen the terminal costs.

One method of locking the customer's laser to a selected frequency isshown in FIG. 7. The coherent customer terminal includes a tunablecoherent source 46 which has a frequency control loop 48 referenced tothe incoming local oscillator frequency detected by a receiver 50connected to the network through a terminal coupler 51. This source islocally tuned to a selected frequency prior to transmission in thenetwork via an optical switch 52. The reference local oscillator isreceived over the network to lock the local intermediate frequency (IF)tunable circuit. The optical switch 52 is then switched to allowtransmission to the network through the terminal coupler 51.

As an alternative to TDMA and FDMA, code division multiple access (CDMA)could be used to allow selective access to each terminal by otherterminals. In this case each terminal has a code correlator whichenables access to the terminal when the code is received from anotherterminal at the start of a message. This has the advantage over TDMAthat it does not require time slots. Indeed, the transparent distributedswitch network is well suited to CDMA as it will allow access to allcustomers, with the correct code, without the need for additionalswitching through a specific network path.

Whereas the abovedescribed couplers have two fibres (18, 20), one ofwhich is provided with light retransmissive means (22) and signalconditioning means (24), the present invention includes within its scopecouplers having more than two fibres, one of which is provided withlight retransmissive means and signal conditioning means asabovedescribed, and wherein at least a further one of the fibres isprovided with a respective light retransmissive means with or without arespective signal conditioning means.

We claim:
 1. An optical coupler disposed as a star node in an opticalsignal distribution network wherein said coupler comprises:a pluralityof light transmissive elements communicatively coupled at a couplingpoint to couple light transmitted in each of the elements into eachother of the elements, one of the light transmissive elements beingprovided with light retransmissive means for reapplying light leavingthe coupler in that element back to the coupling point, said one elementalso being provided with signal conditioning means in the path of thelight transmitted along the said one element; and in which the lightreflective means are at least substantially totally reflective.
 2. Anoptical coupler comprising a plurality of light transmissive elementscommunicatively coupled at a coupling point whereby in use lighttransmitted in each of the elements is coupled into each other of theelements, one of the elements being provided with light retransmissivemeans for reapplying light leaving the coupler in that element back tothe coupling point, characterized in that the said one element is alsoprovided with signal conditioning means in the path of the lighttransmitted along the one element; andin which the signal conditioningmeans are time slot selective.
 3. An optical coupler comprising aplurality of light transmissive elements communicatively coupled at acoupling point whereby in use light transmitted in each of the elementsis coupled into each other of the elements, one of the elements beingprovided with light retransmissive means for reapplying light leavingthe coupler in that element back to the coupling point, characterized inthat the said one element is also provided with signal conditioningmeans in the path of the light transmitted along the one element;inwhich the light retransmissive means are a loop of light transmissivematerial having its ends coupled together and also coupled to the saidone element; and in which the loop is a doped optical fibre loop and inwhich an energy pump is also coupled to the loop.
 4. An optical couplercomprising a plurality of light transmissive elements communicativelycoupled at a coupling point whereby in use light transmitted in each ofthe elements is coupled into each other of the elements, one of theelements being provided with light retransmissive means for reapplyinglight leaving the coupler in that element back to the coupling point,characterized in that the said one element is also provided with signalconditioning means in the path of the light transmitted along the oneelement;in which the light retransmissive means are a loop of lighttransmissive material having its ends coupled together and also coupledto the said one element; and in which a laser amplifier is connected inthe loop.
 5. An optical switching network comprising an optical exchangetransmitter, a plurality of customer terminals, and light transmissiveelements connecting the customer terminals to the exchange transmitter,wherein the network includes:at least one optical coupler for coupling afirst plurality of light transmissive elements to a second plurality oflight transmissive elements at a coupling point, a first element of thefirst plurality of elements being provided with light retransmissivemeans for re-applying light leaving the coupler in that element back tothe coupling point, wherein said first element is also provided withsignal conditioning means in the path of the light transmitted alongsaid first element, whereby light transmitted along a second element ofthe first plurality of elements is coupled into each of the elements ofthe second plurality of elements, whereby light transmitted along anyone of the second plurality of elements is coupled into each of theother elements of the second plurality of elements, and wherein thesecond element of the first plurality of elements connects the exchangetransmitter to the coupler.
 6. A network as in claim 5 in which the oreach coupler constitutes a node in a distributed switch network.
 7. Anoptical coupler for coupling a first plurality of light transmissiveelements to a second plurality of light transmissive elements at acoupling point, a first element of the first plurality of elements beingprovided with light retransmissive means for re-applying light leavingthe coupler in that element back to the coupling point,wherein saidfirst element is also provided with signal conditioning means in thepath of the light transmitted along said first element, whereby lighttransmitted along a second element of the first plurality of elements iscoupled into each of the elements of the second plurality of elements,and whereby light transmitted along any one of the second plurality ofelements is coupled into each of the other elements of the secondplurality of elements.
 8. A coupler as in claim 7 in which the lightretransmissive means are light reflective means arranged to reflectlight in the said one element back to the coupling point.
 9. A coupleras in claim 8, in which at least a further one of the elements isprovided with a respective light retransmissive means with or without arespective signal conditioning means.
 10. A coupler as in claim 8 inwhich the light reflective means reflect the light in the one elementback along it to the coupling point.
 11. A coupler as in claim 8 inwhich the one element is terminated in the light reflective means.
 12. Acoupler as in claim 8 in which the signal conditioning means aredisposed between the coupling point and the light reflective means. 13.A coupler as in claim 8 in which the light reflective means and thesignal conditioning means are constituted by a single device.
 14. Acoupler as in claim 7 in which the signal conditioning means are abi-directional light amplifier.
 15. A coupler as in claim 14 in whichthe amplifier has a pair of light emitting ports, at least one of theports being communicatively connected with the said one element.
 16. Acoupler as in claim 15 in which the other of the ports is coated with alight reflective coating constituting light reflective means.
 17. Acoupler as in claim 7 in which the signal conditioning means include afilter.
 18. A coupler as in claim 17 in which the filter is wavelengthselective.
 19. A coupler as in claim 7 in which the light retransmissivemeans are a loop of light transmissive material having its ends coupledtogether and also coupled to the said one element.